1. Field of the Invention
The present invention relates to a radiation image capturing system for capturing a radiation image of a subject using a radiation conversion panel, and a method of adjusting such a radiation image capturing system.
2. Description of the Related Art
In the medical field, there have widely been used radiation image capturing apparatus which apply a radiation to a subject and guide the radiation that has passed through the subject to an X-ray film or a radiation conversion panel, which captures a radiation image from the radiation.
According to an analog image capturing process which employs X-ray films, the dose and quality (X-ray energy spectrum) of X-rays to be applied to the subject have to be set according to the characteristics of the X-ray film used, in order to produce an image at an optimum density or quality (gradation). Therefore, it is currently difficult to take into account reduction of the level of radiation exposure which the subject undergoes.
There has been developed a radiation conversion panel which is capable of digitally capturing a radiation image. A stimulable phosphor panel (hereinafter referred to as “IP” (Imaging Plate)) is a radiation conversion panel which stores, in a phosphor, radiation energy representative of radiation image information of a subject and which subsequently emits stimulated light representative of the stored radiation image information when irradiated with stimulating light. The IP with the recorded radiation image information is supplied to a reading apparatus, which reads the recorded radiation image information from the IP to produce a visible radiation image from the read radiation image information.
In sites where medical procedures are performed, such as operating rooms or the like, it is necessary to read and display recorded radiation image information immediately from a radiation conversion panel, for the purpose of quickly and appropriately treating the patient. As a radiation conversion panel which meets such a requirement, there has been developed a radiation conversion panel (hereinafter referred to as “FPD” (Flat Panel Detector)) having a solid-state detector therein for converting radiation directly into electric signals, or for converting radiation into visible light with a scintillator, and then converting such visible light into electric signals in order to read the detected radiation image.
There is a radiation image capturing apparatus employing a solid-state imaging device (hereinafter referred to as “CCD” (Charge Coupled Device)) as a radiation conversion panel.
According to a digital image capturing process which employs such a radiation conversion panel, since acquired radiation image information represents digital information, any of various image processings such as density adjustment, gradation adjustment, etc. can be performed on the radiation image information. Therefore, it is possible to make radiation settings about a radiation dose and a radiation quality with a view to reducing the level of radiation exposure on the subject.
It is desirable to appropriately select any one of those different radiation conversion panels capable of digitally capturing radiation images depending on the state of the subject, the location where a radiation image is captured, the purpose for which a radiation image is captured, etc.
For example, the IP is suitable for imaging special regions of an emergency patient or a subject which cannot be moved easily because the IP can easily be carried around, can have a large choice of shapes, and can capture any desired areas of the subject. The FPD lends itself to capturing radiation images of a subject while an operation is being performed on the subject or the subject is undergoing a complete medical checkup because it is possible for the FPD to capture high-quality radiation images quickly. Though the CCD is lower in captured image quality than the IP and the FPD, it is suitable for use in treating the patient using a catheter, for example, while the doctor is confirming radiation images captured by the CCD or for combining a plurality of radiation images captured by the CCD in different directions into a three-dimensional image because the CCD is able to acquire real-time moving images and also to acquire radiation images independent on the direction in which the radiation is applied to the subject.
The radiation conversion panels capable of digitally capturing radiation images have respective inherent sensitivity characteristics. Therefore, appropriate image capturing conditions need to be set up for the respective radiation conversion panels in order for them to acquire suitable radiation image information. The image capturing conditions include, for example, the focal spot size of the radiation source, a filter for adjusting the spectrum of X-rays applied to the radiation conversion panel, a tube voltage, a tube current, an X-ray irradiation time, the type of a target for outputting an X-ray, etc.
Japanese Laid-Open Patent Publication No. 2000-157524 discloses a technique for applying a radiation having a low dose through a subject to a radiation conversion panel to acquire a pre-exposure image of the subject and setting image capturing conditions including a focal track (if the image capturing apparatus has a dual focal track), a filter used, a tube voltage, information as to whether there is a grid or not, a focal spot size, and a value represented by the product of a tube current and an irradiation time, using the acquired pre-exposure image.
According to the technique disclosed in Japanese Laid-Open Patent Publication No. 2000-157524, however, the radiation conversion panel used is predetermined, and image capturing conditions for acquiring appropriate radiation image information can be set with respect to only the radiation conversion panel. The disclosed technology is not directed to the setting of image capturing conditions suitable for a selected one of different radiation conversion panels.